Hop material is added to beer during the brewing process in the form of whole hops, (enriched) hop powder, pellets or as an extract, giving the beer the typical hop bitterness.
Because of the considerable reduction in volume an extract can be more easily transported, more cheaply stored and, because of its good keeping quality, can economically compensate for harvest fluctuations. Moreover, an extract can be readily dosed.
By reason of these advantages hop extracts are now well known in the art. Today, primarily hydrocarbons, chlorinated hydrocarbons and alcohols, e.g. hexane, methylene chloride and ethanol are used as solvents. Nonpolar solvents mainly provide the resin components. Therefore, an aqueous extraction has to follow in order to extract the tannins. With polar solvents, e.g. ethanol, the extract contains already parts of the tannins.
The solvents must, as far as possible, be removed from the pasty extracts. Though the residual solvent content is very low (e.g. with methylene chloride below 0.1%), one must remember that the suspicion that methylene chloride is toxic or cancerogenic, respectively, has not yet been completely invalidated. Thus, tests have recently been made to extract hops with a physiologically harmless solvent, namely with compressed liquid carbon dioxide.
Under ambient conditions gases do not have solvent properties. However, under increased pressure, i.e. in liquified or compressed conditions, respectively, gases may be used as solvents. Below the critical temperature (31.degree. C.) gaseous carbon dioxide can be compressed by increasing pressure and liquified.
By a further increase of pressure the liquid may (to a minor extent) be further compressed. Above the critical temperature the transition from the gaseous into the compressed condition is continuous when pressure is increased, i.e. without discontinuous increase of density as is the case with the compression below the critical temperatue.
DE-OS No. 28 01 843 discloses an extraction process for hop substances with liquid carbon dioxide, where liquid carbon dioxide is passed through a column of hop material at -5.degree. to +15.degree. C. whereby at least part of the alpha acids contained in the hops is extracted into the liquid carbon dioxide and a hop extract of high purity is then obtained from the liquid carbon dioxide.
Using compressed liquified carbonic acid (i.e. the carbonic acid is subcritical with respect to temperature and supercritical with respect to pressure) the hop substances may be treated in accordance with DE-AS No. 28 27 002.
Finally, from DE-PS No. 21 27 618, it is known to extract hops with CO.sub.2 which is supercritical with respect to pressure and temperature. Extraction pressures between 100 and 400 bar and extraction temperatures of up to 100.degree. C. are mentioned. On the other hand, (column 4, line 54, et seq.) it is also stated that the thermic sensitivity of the material has to be taken into account and therefore the extraction is preferably carried out between 40.degree. and 50.degree. C.
There is, in fact, a prejudice against submitting natural substances, such as hops, to high temperatures because of chemical reactions which could cause damage.
In several works concerned with changes in substances by reason of the increase of temperature on drying, storing and pelletizing, it has been shown that increased temperatures do indeed cause a degradation of important substances.
In fact, however, the initial extraction with organic solvents is normally followed by an aqueous extraction of the water-soluble substances at temperatures near the boiling temperature of water. However, this additional extraction does not disprove the above described prejudice against high temperatures, because the sensitive substances have already been removed.
Neither is this prejudice disproved by an extraction method described in DE-PS No. 4 89 719. In this extraction the steam-distillable substances are distilled off at 120.degree.-130.degree. C., then the residue product is boiled out in a sugar solution. The thus obtained product is more an aqueous extract and differs in its composition from the resin extract which was obtained with organic solvents or CO.sub.2, respectively.
Finally, according to a publication by E. Kruger ("The use of CO.sub.2 in the manufacture of hop products") in "Monatsschrift fur Brauerei" 33, No. 3 (1980), which describes the optimal parameters for the hop extraction at temperatures between 21.degree. and 80.degree. C., the yeild of alpha acids does not rise continuously with raising temperature, but on the contrary, from a certain temperature level upward it suddenly decreases.